This application claims the priority of German Application No. 198 38 222.7, filed Aug. 22, 1998, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a method for evaluating an ion current signal of a self-igniting internal combustion engine.
A method is already known for evaluating an ion current signal of a self-igniting internal combustion engine (WO 86/00961), with a measured ion current signal being integrated to determine the carbon content, for example. The rpm can be determined from the spacing of the measurement signals with time.
The goal of the present invention is to expand the evaluation capabilities of an ion current signal.
This goal is achieved according to the invention by a method for evaluating an ion current signal of a self-igniting internal combustion engine, characterized in that a local maximum in the curve of the ion current signal relative to an injection point in time in the ignition shift can be derived from time shift.
The ion current signal can be appropriately windowed in order to be able to detect local maxima during signal evaluation. With this method, the ignition shift both during preinjection and during main injection can be detected.
In a further method according to the invention, the parameters of the main injection can be controlled as a function of an evaluation of the measured ion current following a preinjection.
When controlling the parameters of a main injection on the basis of a measured ion current of the previous main injection, it can happen that a main injection takes place with a fuel volume that is not optimal or with an injection point in time that is not optimal. These parameters cannot be corrected until the next main injection. Advantageously, in this method according to the invention, the main injection can be corrected prematurely, immediately after evaluation of the preinjection and a parameter possibly recognized as not being optimal.
Such a parameter can be, for example, the reacted preinjection volume. For example, the pre-mixed components of the main injection can be controlled.
In the inventive method, there is also an adaptation of the parameters of the main injection as a function of the measured ion current after at least one previous main injection.
This method describes a procedure in which, during evaluation, the measured ion currents of previous main injections are jointly evaluated with appropriate weighting with the measured ion current following a preinjection.
In a further method according to the invention, a carbon determination is performed by an evaluation of the time curve of the ion current signal following an injection.
It has been found that comparatively accurate carbon determination is possible with an evaluation of the time curve on the ion current signal.
In the method according to the invention, the evaluation advantageously takes place with the time curve being approximated by determining the parameters of an e-function that falls with time.
By determining these parameters, a carbon determination can be performed at comparatively low expense during evaluation. The parameters can be determined, for example, using the methods known from parameter identification in control technology.
In the method according to the invention, excessive carbon formation is advantageously detected when at least one of the parameters differs from a previous setpoint by more than a certain amount.
In the method according to the invention, excessive carbon buildup is advantageously detected when, during the determination of the deviations of the measurement points from the curve produced by the parameters, an error is found in the determination of at least one parameter that is larger than a certain value.
In determining the parameters, they are calculated from the measurement points. Because of the number of measurement points, a system of equations is obtained. The parameters are then determined in such fashion that a curve is produced to which the measurement points fit as optimally as possible. In the case of the parameter identification mentioned above, the parameters are determined, for example, by the least squares technique. This provides an indication of how well the measurement points are represented by the curve. When the error becomes too great, it is clear that no parameters can be determined logically any longer.
In the present application, it turns out that the measurement points of the ion current with excessive carbon formation no longer can be logically approximated by an e-function. In determining parameters by means of parameter identification, this can be detected for example by the least squares method. When the error exceeds a certain values, excess carbon formation can be detected.
In a further method according to the invention, continuous injection is detected in a cylinder with the ion current signal being integrated and evaluated in terms of area and/or with the position and/or height of the maximum of the ion current signal being evaluated.
During continuous injection, it turns out that the area below the ion current signal increases. In addition, it develops that the maxima of the ion current signal shifts and change their heights. Advantageously, therefore, such a continuous injection can be detected using the method wherein depending on an evaluation of the measured ion current following a preinjection, the parameters of the main injection can be controlled.
The use of this method proves to be especially advantageous in a vehicle with a self-igniting internal combustion engine, in which the fuel is supplied to the individual cylinders. The fuel is kept in a pressure reservoir. This pressure reservoir is connectable by valves with the individual cylinders. Such systems are known as common-rail systems. If one of these valves sticks in the open position, fuel is fed continuously to that cylinder. Advantageously, this can also be detected by the method according to the invention, wherein continuous injection in a cylinder is detected when the ion current signal is integrated and evaluated in terms of area and/or when the position and/or height of the maximum of the ion current signal are evaluated.
In addition, the ion current signal can also be evaluated as to whether the area below the curve is smaller than a certain threshold value. In this case, it can be concluded that no fuel is being supplied.
In the method according to the invention, injection is advantageously suppressed at least into the cylinder in which continuous injection is detected.
As a result, destruction of the internal combustion engine can advantageously be prevented. It may be advantageous to shut off more than one cylinder to improve the operating smoothness of the engine.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.